HDRMerge AppImage - first experiment

Here is a new version of the AppImage, this time correctly using LibRaw 0.17.2:

I also discovered a mistake in the HDRMerge sources: in two places, the LibRaw header is included like that:

#include <libraw/libraw.h>

However, the correct syntax should be

#include <libraw.h>

because the cmake configuration already sets

-I${prefix}/libraw

in the compiler options. As a consequence, the current way of including LibRaw always picks the system-wide header in

/usr/include/libraw/libraw.h

even when a custom version is being linked.

@heckflosse: to which github issue tracker should I file this bug?